The present invention relates to a method of processing exhaust gases produced by a semiconductor manufacturing process in which at least hexafluoroethane is concentrated for subsequent recovery by adsorbing oxygen or both oxygen and nitrogen present within the off-gases.
Perfluorocarbons, such as hexafluoroethane (C.sub.2 F.sub.6), are used in the semiconductor industry for cleaning purposes. Hexafluoroethane is fed with oxygen, generally in equal parts by volume, into a semiconductor fabrication chamber. Carbon tetrafluoride (CF.sub.4) is produced as a by-product. Since nitrogen is used as a purge gas to vacuum pumps, the effluent or exhaust gases from the chamber can contain carbon tetrafluoride, hexafluoroethane, oxygen, and nitrogen. Additionally unconsumed etchants such as nitrogen trifluoride, can also be present. Since perfluorocarbons, such as the carbon tetrafluoride or hexafluoroethane, present an environmental hazard as contributing to greenhouse effect, they cannot be discharged to the atmosphere and must be recovered or abated. With respect to hexafluoroethane, recovery presents a more attractive option than abatement because the recovered hexafluoroethane can be recycled.
Commonly, the effluent or off-gas is recovered from a chamber by pumping with a vacuum pump. In order to aid in the pumping operation and in some cases to prevent potential flammable substances from concentrating, the exhaust gases are diluted with nitrogen. Since the effluent also contains acid gases and other undesirable species, typically the exhaust gases are scrubbed before any further treatment. The perfluorocarbon content of the resultant feed stream is then recovered by adsorption and membrane techniques. For instance, in EP 0754487, perfluorocarbons are recovered by passing an effluent stream into a membrane unit selected such that the perfluorocarbon content forms the non-permeate stream while the carrier gas content permeates the membrane and thus forms the permeate stream. However, at low concentrations the membrane area required and the compressor requirements may not be cost effective. In known adsorption techniques, the perfluorocarbons are recovered at low pressure. A problem with adsorption is that the resultant product stream, must be recompressed before further usage or treatment.
In a recent patent EP 819779, a multi-port valve is positioned between the scrubber and one or more treatment chambers so that etchant streams may be diluted and then recovered or abated and cleaning streams that contain the perfluorocarbons of interest here are segregated from other process gas streams and then routed directly for treatment without any dilution or with slight dilution with added nitrogen to serve as a ballast gas to promote the pumping of the off-gas. As such, it is contemplated in this patent that the resultant feed to be treated has a higher concentration of hexafluoroethane (as well as other perfluorocarbons) than is contemplated in prior art methods.
As will be discussed, the present invention has direct application to treatment apparatus and methods in which a feed to be treated is generated in the manner contemplated by EP 819779 and further, has the advantage for any treatment in that the recovery of the hexafluoroethane is not conducted at reduced pressure.